The present invention relates to a device and a method for controlling the weft winding arm in weft feeders for weaving looms.
Conventional weft feeders are devices which comprise a fixed drum on which a weft winding arm, which rotates around the drum, winds in a fishing-reel fashion a plurality of turns of yarn which constitute a weft reserve. The turns unwind from the drum of the feeder, when requested by the loom, at each weft insertion, and the weft winding arm, under the control of a supervisor microprocessor, rewinds the turns in order to restore the weft reserve.
Currently, the motor most widely used for the movement of the weft winding arm is of the asynchronous three-phase type. This choice is essentially dictated by the inherent characteristics of these motors and mainly by their low manufacturing and maintenance costs, which are allowed by their simple and sturdy structure and by the complete lack of elements in mutual sliding contact.
Moreover, the evolution of semiconductor technology has provided supervisor microprocessors which integrate peripheral units capable of directly generating the waveforms of the control signals for the inverter that drives such motors, where the term "inverter" is used to designate the driving device capable of generating a system of multiphase sinusoidal voltages with freely variable amplitude and frequency.
However, although the asynchronous motors are satisfactory in performance/cost terms, they suffer considerable drawbacks which limit the performance, especially in application to the movement of the weft winding arm of weft feeders.
The main drawback is the impossibility to achieve simple and effective control of the torque delivered by the motor. To do this, it is in fact necessary to resort to sophisticated control systems of the vector type which however, due to their complexity and to their high computing power requirements, are not adapted to be installed at low cost in weft feeders. Accordingly, it is necessary to renounce to these expensive and complicated control systems and to assume an open-loop system for the adjustment of the speed of the motor, i.e. the synchronization speed set by the inverter is chased by the motor.
In this manner, however, it is impossible to obtain high dynamic performance from the motor, and this is a severe drawback if the weft winding arm of the weft feeder is required to perform high accelerations and decelerations, as occurs increasingly often as a consequence of the continuous increase in the weaving speed.
Moreover, with the open-loop adjustment system the current absorbed by the motor is often significantly higher than the current that is actually necessary; accordingly, the power absorbed in excess is dissipated as heat, is and this causes harmful overheating of the motor and of the electronic components of the power section of the inverter.